The oil sands in Northern Alberta constitute one of the largest hydrocarbon reserves in the world. Oil sands are a combination of bitumen, quartz sand, clay, water and trace minerals. Bitumen can be recovered from oil sands using two main methods: open-pit mining and in situ drilling. Approximately 20% of the oil sands lie close enough to the earth's surface to be mined.
The key characteristic of Alberta oil sand that makes bitumen economically recoverable is that the sand grains are hydrophilic and encapsulated by a water film which is then covered by bitumen. The water film prevents the bitumen from being in direct contact with the sand and, thus, by slurrying mined oil sand with heated water, the bitumen is liberated from the sand grains and moves to the aqueous phase. However, the composition of oil sands varies from deposit to deposit and the recovery of bitumen from a particular deposit will depend on a number of factors including the grade of the oil sand, i.e., the bitumen content, the fines content, the connate water chemistry, the minimum mining thickness, and the ratio of total volume to bitumen in place. Hence, various processing conditions have been developed for successful extraction of bitumen from oil sands, which processing conditions will be discussed in more detail below.
It is well understood in the industry that the quality of the oil sand has very significant effects on bitumen recovery. For example, a “low grade” oil sand typically contains between about 6 to 10 wt. % bitumen with greater than about 25 wt. % fines. An “average grade” oil sand typically contains at least 10 wt. % bitumen to about 12.5 wt. % bitumen with about 15 to 25 wt. % fines and a “high grade” oil sand typically contains greater than 12.5 wt. % bitumen with less than 15 wt. % fines. Fines are generally defined as those solids having a size less about 44 μm. The higher fines concentration in low to average grade oil sand contributes to the difficulty in extracting the bitumen.
Further, final mine pit limits are also influenced by physical limits such as lease limits, roadways, river courses, plant facilities, and associated necessary geotechnical offsets. The requirements for power lines, pipeline corridors, communication lines, ditches, heavy equipment haul roads, light vehicle access roads, etc. are all incorporated into mining limits. Thus, generally, each mine pit (site) will have its own individual unique limitations to overcome.
In view of all of the above, several different bitumen extraction processes have been developed to deal with variations in oil sand ore at various mine sites as well as other limitations as listed above. An oil sands bitumen extraction process generally includes the following steps: preparing an oil sand and water slurry from mined oil sand (slurry preparation), conditioning the oil sand slurry (slurry conditioning), and subjecting the oil sand slurry to a separation process to recover the bitumen (bitumen separation) (collectively referred to generally as “bitumen extraction process”).
As used herein “slurry preparation” means the preparation of a water and oil sand slurry in a slurry preparation unit. As used herein, “slurry conditioning” means the digestion of oil sand lumps present in the oil sand slurry, liberation of bitumen from sand-fines-bitumen matrix, coalescence of liberated bitumen flecks into larger bitumen droplets and aeration of bitumen droplets. As used herein, “bitumen separation” means the separation of bitumen from the solids and water present in the conditioned oil sand slurry, commonly in a separation vessel such as a gravity separator.
One bitumen extraction process commonly used in the industry is referred to herein as the “hot water process”. In general terms, the hot water process involves feeding the mined oil sand into a rotating tumbler where it is mixed for a prescribed retention time (generally in the range of 2 to 4 minutes) with hot water (approximately 80-90° C.), steam, caustic (e.g., sodium hydroxide) and naturally entrained air to yield a slurry that has a temperature typically around 80° C. The bitumen matrix is heated and becomes less viscous. Chunks of oil sand are ablated or disintegrated. The released sand grains and separated bitumen flecks are dispersed in the water. To some extent bitumen flecks coalesce and grow in size. They may contact air bubbles and coat them to become aerated bitumen (slurry conditioning). Thus, in the hot water process, both oil sand slurry preparation and slurry conditioning occur in the tumbler.
The conditioned slurry is introduced into a separation vessel typically operating at 55 to 80° C. to recover the bitumen. One of the limitations of the hot water process is that, in general, such a tumbler based plant is at a fixed location, ideally, one where large amounts of hot water/steam can be produced.
The hot water process generally produces good bitumen recoveries for all grades of oil sand. However, the thermal energy requirement per tonne of oil sand processed is very high. In particular, thermal energy is required to heat the process water, for steam production and for heating the flood water. Thus, the hot water process may only be practiced at particular mine sites due to such limitations.
Another bitumen extraction process, which is disclosed in Canadian Patent No. 2,029,795 and U.S. Pat. No. 5,039,227, involves the use of a pipeline to condition oil sand slurry. In this process, heated water (typically at 95° C.) is mixed with the dry as-mined oil sand at the mine site in predetermined portions using a device known as a “cyclofeeder”, to form an aerated slurry having a temperature in the range of 40-70° C., preferably about 50° C. (slurry preparation). The oil sand slurry is then conditioned through several kilometers of pipeline (slurry conditioning) and transported to an extraction plant where bitumen separation occurs typically at 55° C. in a separation vessel. This extraction process is referred to herein as the “warm slurry process”.
Because of the use of a hydrotransport pipeline to condition the oil sand slurry, the warm slurry process allows for more flexibility, e.g., the mine site may be more remotely located from a bitumen separation plant where bitumen froth is produced from the conditioned oil sand slurry. Furthermore, in warm slurry extraction, the slurry preparation unit is generally relocatable and can be moved when required. The hydrotransport pipeline which is used for conditioning can also be moved when required.
Thus, in the warm slurry process, the pumping of the slurry through a pipeline, over a certain distance, allows the slurry to be conditioned at a lower temperature of about 50 to 55° C. With increased conditioning time (i.e., typically 10 minutes or greater) in the pipeline, this process does not compromise conditioning and bitumen recovery. Further, this process allows the slurry preparation at the mine site and the bitumen separation at the bitumen separation plant, thereby reducing the requirement of dry oil sand transportation. Hence, the warm slurry process generally has a reduced carbon footprint and a reduced energy requirement.
In some instances, for example, at very remote mine sites where access to thermal energy is limited, it is desirable to reduce the thermal energy requirement per tonne of oil sand even more. Thus, an even lower energy consuming bitumen extraction process was developed, which is disclosed in Canadian Patent Nos. 2,217,623 and 2,506,398, and which is hereinafter referred to as the “low energy process”, i.e., a process where slurry preparation and conditioning typically results in an oil sand slurry having a temperature in the range of about 40-55° C. The low energy process involves mixing the mined oil sand with water having a temperature of about 75-85° C. in predetermined proportions in a mix box located near the mine site to produce a slurry containing entrained air and having a controlled density in the range of 1.5 to 1.6 g/cc. The slurry is then pumped through a pipeline to condition and transport the slurry (slurry conditioning). The separation of bitumen from the conditioned slurry typically occurs at about 35° C.
As mentioned, this process is particularly useful for mine locations where there is limited access to hot water and steam and, in particular, at remote mine locations. Because hot water is heated locally, i.e., requiring a power generation system, a mine site can be located far away from the base plant where bitumen froth cleaning and upgrading take place.
It is understood that other slurry preparation units can be used, such as the unit described in Canadian Patent Application No. 2,480,122. When using this slurry preparation unit, little or no rejects will be produced during slurry preparation. The slurry preparation unit comprises a series of roll crushers spread vertically throughout a portion of a slurry preparation tower. The slurry preparation tower typically uses gravity to move the oil sand through the tower. Typically, each roll crusher is made up of a number of crusher rolls spaced a certain distance apart to reduce the size of large pieces of oil sand before the lumps of oil sand drop through the crusher rolls to the next roller crusher beneath or at the bottom of the slurry preparation tower. Each successively lower roll crusher reduces the lumps of oil sand even smaller until the oil sand is fine enough to form a pumpable oil sand slurry.
At the same time the oil sand passes through the different roll crushers, heated water is added to the oil sand to form a slurry. Typically, the stream of oil sand passing through the levels of roll crushers is sprayed with the heated water, as it passes down the tower. The mixing of this oil sand with the streams of hot water will form the eventual oil sand slurry, which is typically received in a pump box for feeding the slurry to a pump and pipeline system. This process reduces the bitumen loss to the rejects due to the decreased amount of rejects, thus allowing more bitumen to be recovered. This process is particularly useful when it is desirable to produce minimal rejects and is hereinafter referred to as the “wet crushing slurry preparation process”.
In summary, selection of a particular slurry preparation process, slurry conditioning process, and bitumen separation process will depend on a number of factors, including the remoteness of the mine site, the ability and cost to truck mined oil sand to the slurry preparation units and the energy availability at the mine site.